1. Field of the Invention
The present invention relates to the field of nuclear magnetic resonance (NMR), and more particularly to structural analysis, including but not limited to structural analysis of living tissue using modifications of NMR techniques.
2. Prior Art
The following terms will be used throughout the text:
1. Prism: A prism is an elongate volume of material from which a signal is measured. The signal varies as a function of position along the prism. Although the form of prism generally referred to in the text has a rectangular cross section, in general it may include shapes of any arbitrary cross-section. Additionally, although the measurement of signal generally referred to in the text is a modification of current MRI techniques, as it is evident to anyone skilled in the art, the signal could also be gathered from a prism volume using other techniques.
2. Profile: A profile is the signal as a function of position along a single direction within a prism. Profiles taken from materials with heterogeneous signal intensity will in general be non-constant and represent a measure of the amount of signal generating material with position in the prism. As an example, this may be presence or absence of material, or material density. However, it could measure any number of properties of the material, but the salient point that the signal versus position gives a measure of some physical property with position along the prism. As is evident to one skilled in the art, the profile could represent any measurable physical property which varies with position.
3. Spatial frequency: In signal processing, many techniques exist for estimating the frequency content of a signal. Generally these signals vary with time. When these techniques are applied to a signal which varies with position, rather than time, the analogue of frequency content is a measure of the frequency of spatial variations along the profile. For the purposes of this discussion, this will be termed the “spatial frequency”.
4. Spatial frequency spectrum: A spatial frequency spectrum is a representation of the spatial frequency content of a segment of a profile. It graphically illustrates the relative or absolute amounts of signal present in the segment as a function of spatial frequency.
U.S. Pat. No. 7,932,720, the disclosure of which is hereby incorporated herein by reference, discloses methods of assessing at least one spatial frequency characteristic of a sample of a structure. A representative embodiment of the method can be summarized as: a) subjecting the sample to a magnetic field, b) subjecting the sample to first and second RF excitations to excite a stick shaped region in the sample, c) receiving an echo signal from the stick shaped region while the sample is subjected to a magnetic field gradient, d) taking an inverse transform of the received echo signal to obtain an echo signal intensity in one dimension, e) identifying a region of interest in the echo signal intensity in one dimension, f) windowing the region identified in e) along the stick shaped region to shape the echo signal intensity in the region of interest along the stick shaped region, g) taking a transform of the windowed echo signal intensity in one dimension obtained in f), and h) analyzing the one dimensional spatial frequency content in the transform obtained in g) in order to access a one dimensional spatial frequency distribution within the sample of the structure without having to acquire all of the data required to generate an image generally referred to herein as spatial frequency spectroscopy.
Spatial frequency spectroscopy has the significant advantage of being able to characterize structures at significantly smaller scales than can be achieved using conventional MR imaging techniques. By way of example, in some tissues or disease states of interest, including but not limited to brain tissue, the prism cross-section is required to be made small (for example on the order ˜1 mm). The conventional 90 and 180 degree pulses defining these small ˜1 mm prism cross-section dimensions are particularly susceptible to effects caused by deviations from a pure 90 or 180 degree pulse during the 90 and 180 slice selection process to define the stick shaped or prism shaped region of interest. These imperfections may cause undesirable artifacts in the data. Smaller cross section prisms suffer more from this effect for at least two reasons. First the signal-to-noise ratio is lower, and second, thin slice selective pulses require more time to produce a reasonable slice selective excitation and the proportion of edge effects to total slice thickness is less favorable than in a slice several mm thick.
The Problem
Selective excitation of inner volume prisms is accomplished in MRI scanners by applying two intersecting slice selective RF pulse excitations in the presence of gradients (see U.S. Pat. No. 7,932,720, the disclosure of which is hereby incorporated herein by reference). The first slice selective excitation is a 90° pulse while the second is a 180° pulse. When the prisms are of sufficiently small cross section, or the slices selected by the gradients are sufficiently thin (less than 4 mm) the following issue is encountered: The profile of the 180° pulse slice select deviates significantly from an ideal rectangular intensity profile (a diagram of the 180° pulse slice select shape is shown in FIG. 1). The thinner the slice gets, the more the slice profile differs from rectangular. This leads to there being a non-trivial portion of the intended 180° slice selection volume being excited by other than a simple refocusing 180° pulse. The material in this off 180° condition will then have a non-trivial transverse magnetization which will produce a free induction decay signal. This is detrimental for at least two key reasons—the first is that material potentially distant from the intended prism volume will produce contributions to the echo signal and corrupt the signal from the intended prism volume. Secondly the signal from this unintentionally excited material will produce “encoded” signals in the echo when the read gradient is applied to disperse the spatial frequencies in the intended direction. This material excited by the imperfections in the 180° pulse has not been pre encoded, so when the receiver is turned on and the read gradient is applied, the material within the prism is encoded as intended at spatial frequencies starting at −k-max and progressing to +k-max, and simultaneously the imperfectly excited material in the 180° selected slice is initially encoded at k=0 (zero spatial frequency) and rises as a function of time. Since a k=0 encoded signal is significantly higher in magnitude than non-zero encodes, it produces a large signal at the beginning of the read, corrupting the signal from the prism volume.
FIG. 2 illustrates in detail the pulse sequence and illustrates the problem described above. FIG. 3 presents a schematic diagram illustrating the relative orientations of the 90° and 180° slice selections.
The spatially encoded echo signal from this material is superimposed on the spatially encoded echo signal from the material of interest inside the prism but with different spatial encodes at any given point in time. The large peak at −k-max spatial frequency leads to the impression of a large exponential decay at high negative frequency on the leading edge of the echo. See FIG. 4 for an example of an echo contaminated with FID. This phenomenon is referred to herein as FID leak through.
The invention described here is a method to reduce or eliminate this effect while simultaneously providing for a good signal to noise ratio and retain the ability to incorporate a range of MRI contrast mechanisms.